// 图片压缩
export const imgPreview = (file, callback) => {
  //将base64转换为文件
  function dataURLtoFile(dataurl, file) {
      var arr = dataurl.split(","),
          bstr = atob(arr[1]),
          n = bstr.length,
          u8arr = new Uint8Array(n);
      while (n--) {
          u8arr[n] = bstr.charCodeAt(n);
      }
      return new File([u8arr], file.name, {
          type: file.type
      });
  }
  // 压缩图片
  function compress(img) {
      let canvas = document.createElement("canvas");
      let ctx = canvas.getContext("2d");
      //瓦片canvas
      let tCanvas = document.createElement("canvas");
      let tctx = tCanvas.getContext("2d");
      // let initSize = img.src.length;
      let width = img.width;
      let height = img.height;
      //如果图片大于四百万像素，计算压缩比并将大小压至400万以下
      let ratio;
      if ((ratio = (width * height) / 4000000) > 1) {
          // console.log("大于400万像素");
          ratio = Math.sqrt(ratio);
          width /= ratio;
          height /= ratio;
      } else {
          ratio = 1;
      }
      canvas.width = width;
      canvas.height = height;
      //    铺底色
      ctx.fillStyle = "#fff";
      ctx.fillRect(0, 0, canvas.width, canvas.height);
      //如果图片像素大于100万则使用瓦片绘制
      let count;
      if ((count = (width * height) / 1000000) > 1) {
          // console.log("超过100W像素");
          count = ~~(Math.sqrt(count) + 1); //计算要分成多少块瓦片
          //      计算每块瓦片的宽和高
          let nw = ~~(width / count);
          let nh = ~~(height / count);
          tCanvas.width = nw;
          tCanvas.height = nh;
          for (let i = 0; i < count; i++) {
              for (let j = 0; j < count; j++) {
                  tctx.drawImage(
                      img,
                      i * nw * ratio,
                      j * nh * ratio,
                      nw * ratio,
                      nh * ratio,
                      0,
                      0,
                      nw,
                      nh
                  );
                  ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
              }
          }
      } else {
          ctx.drawImage(img, 0, 0, width, height);
      }
      //进行最小压缩
      let ndata = canvas.toDataURL("image/jpeg", 0.5);
      tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;
      return ndata;
  }
  // 看支持不支持FileReader
  if (!file || !window.FileReader) {
      return;
  }
  if (/^image/.test(file.type)) {
      // 创建一个reader
      let reader = new FileReader();
      // 将图片转成 base64 格式
      reader.readAsDataURL(file);
      // 读取成功后的回调
      reader.onloadend = function() {
          let result = this.result;
          let img = new Image();
          img.src = result;
          //判断图片是否大于500K,是就直接上传，反之压缩图片
          if (this .result.length <= 200 * 1024) {
              // 上传图片
              let imageFile = dataURLtoFile(this.result, file);
              callback(imageFile);
          } else {
              img.onload = function() {
                  let data = compress(img);
                  // 上传图片
                  let imageFile = dataURLtoFile(data, file);
                  callback(imageFile);
              };
          }
      };
  }
};